1. Field of the Invention
The present invention relates to an adhesive composition. More specifically, the present invention relates to an adhesive composition suitable for bonding wood, in particular hardwood. The present invention also relates to process for preparing the adhesive composition and use of the same.
2. Description of the Related Art
Vinyl acetate-ethylene (VAE) copolymer-based emulsions have been widely used in two-package adhesive compositions. In the conventional processes for preparing VAE copolymer-based emulsions, polyvinyl alcohols are typically used as protective colloid to maintain the stability of the emulsions. However, the emulsions obtained from those processes generally have relatively large particle size. As a consequence, the wood adhesive compositions obtained therefrom are only suitable for bonding wood having low density and hardness; whereas for wood having higher density and hardness, such as beech, birch, oak or the like, the bonding strength and water resistance are generally poor. For example, when used for bonding bench, the compression shear strength after boiling is typically only about 3.5 to 4.5 MPa, and the initial compression shear strength after applying is typically only about 4 MPa, measured according to JIS K6806, which cannot meet the requirements for high quality solid wood mosaic.
Japanese Patent Application Laid-Open No. 2003-171634 discloses an adhesive composition, comprising: (A) an ethylene-vinyl acetate copolymer emulsion, in which vinyl alcohol-based polymer having a saponification degree of 70 mol % or higher is used as dispersing agent, (B) a polyisocyanate compound, and (C) a vinyl alcohol-based polymer, wherein the amount of unreacted vinyl acetate monomer is 0.2 wt % or less and the concentration of formaldehyde is 1 ppm or less. According to that application, sufficient boiling water resistance may not be obtained when the level of the vinyl alcohol-based polymer, as dispersing agent, is lower than 2 parts by weight or higher than 15 parts by weight, per 100 parts by weight of the ethylene-vinyl acetate copolymer-based resin.
Japanese Patent Application Laid-Open No. 2003-082320 describes an aqueous adhesive composition, comprising: as base resin, a mixture of a resin emulsion and a polyvinyl alcohol, and as curing agent, a mixture of a high molecular weight compound and a low molecular weight compound both having at least two isocyanate groups. That application mentions, as examples of the resin emulsion, acrylic resin emulsion, vinyl acetate acrylate emulsion, ethylene-vinyl acetate resin emulsion, styrene-butadiene copolymer, methyl methacrylate-butadiene copolymer, styrene-butadiene-methyl methacrylate copolymers and polyisoprene. That application aims at the improvement of the curing agent, instead of the dispersing agent.
The compositions according to the above literatures are not suitable for bonding wood with higher density and hardness, as they are totally silent about the control of the particle size of the emulsions.
Thus, despite the foregoing efforts, a need still exists for an adhesive composition that have one or more of the following properties: suitable for bonding wood, in particular hardwood; having desirable high compression shear strength after boiling and initial compression shear strength after applying.